FIG. 1 of the present specification shows a simplified linear depiction of a signal transduction pathway that leads to cellular proliferation. This pathway is referred to herein as the "Ras signaling pathway" because Ras is a central relay in this pathway, receiving signals from upstream elements (e.g., growth factor receptors) and transmitting them to downstream elements.
The signaling pathways initiated by growth factor receptors which lead to cellular proliferation, and in some cases malignant transformation, are being elucidated. Many growth factor receptors such as those for epidermal growth factor (EGF) and platelet-derived growth factor (PDGF), as well as EGF receptor-related molecules (e.g. Her-2/Neu/ErbB2), possess an intrinsic tyrosine kinase activity which is activated by ligand-induced receptor dimerization (Heldin, 1995). This results in autophosphorylation of the receptor on tyrosine residues and the binding of proteins containing Src-homology 2 (SH2) domains. Two such SH2 proteins are Grb2 and SHC which indirectly activate the plasma membrane-associated, small GTP-binding protein Ras. Ras activation also occurs in response to ligand binding to seven transmembrane domain G-protein coupled receptors (e.g. Gutkind, 1998). Activation of Ras and other growth factor receptor-regulated signaling pathways ultimately leads to changes in the cytoskeleton and gene expression which are necessary for cellular proliferation, differentiation, and transformation (reviewed in Campbell et al., 1998).
The 3 human ras genes (Ha-Ras, N-Ras, and Ki-Ras) encode 4 proteins (due to alternative splicing of the Ki-Ras mRNA). Under normal circumstances, Ras proteins cycle between an active (GTP-bound) state and an inactive (GDP-bound) state. Ras activation occurs by exchange of bound GDP for GTP, which is facilitated by a family of guanine nucleotide exchange factors. Ras inactivation occurs by hydrolysis of bound GTP to GDP. This reaction is facilitated by GTPase activating proteins (GAPs) (Trahey and McCormick, 1987). In many human cancers, Ras proteins become oncogenically activated by mutations which destroy their GTPase activity, and thus deregulate Ras signaling (reviewed in Campbell et al., 1998).
Multiple candidate Ras effectors exist that may serve downstream of Ras in signal transduction and oncogenic transformation, including members of the Rho family of small GTPases, phosphatidylinositol-3 kinase (PI3K) and the serine/threonine protein kinase c-Raf-1 (reviewed in Campbell et al., 1998). Raf-mediated signaling is the best characterized Ras effector pathway. Activated Ras recruits Raf to the membrane where Raf activation occurs. Activated Raf is the initial component of a kinase cascade, the Mitogen-Activated Protein Kinase (MAPK) cascade (reviewed in Lowy and Willumsen, 1993; Campbell et al., 1998). Raf phosphorylates and activates the MEKI and MEK2 (MAPK/ERK kinase) protein kinases which, in turn, phosphorylate and activate the Extracellular signal Regulated Kinases ERK1 and ERK2 (also known as MAPK1 and MAPK2). Unlike their downstream targets, ERK1,2, the MEK1,2 proteins are highly specific enzymes whose only known substrates are the ERK1,2 proteins. Upon activation, ERK1 and ERK2 phosphorylate (and thus regulate) a variety of target proteins, including nuclear transcription factors, leading to the ultimate cellular response. This linear pathway of Ras signaling is diagrammed in FIG. 1.
The importance of these signaling pathways in the abnormal growth of cancer cells is indicated by the finding that growth factor receptor and Ras pathway components are often mutated and/or overexpressed in cancer. For example, Ras is mutationally activated in about 30% of human cancers including a high percentage of major epithelial cancers such as lung, colon and pancreatic cancers. Additionally, overexpression of growth factor receptors occurs in a number of cancers (e.g. overexpression of the Her-2/Neu receptor occurs in about 30% of human breast cancer). These observations have led to the pursuit and development of agents designed to block individual components of either signal transduction pathway. While such agents hold potential as novel cancer therapeutics, many inhibitors of signal transduction are thought to act in a cytostatic rather than a cytotoxic fashion by blocking the cell's progression through the cell cycle. This distinguishes them from traditional cancer chemotherapy drugs in being less toxic but also possessing less dramatic antitumor activity.
Therefore, there remains a challenge to provide new and improved methods of treating cancer. For instance, to treat tumorigenic cancer cells, it would be highly desirable to provide new methods that achieve a dramatic and selective induction of cancer cell death while minimizing potential toxic side effects against normal, untransformed cells. The present invention provides just such methods of treatment.